> "Could bananas be common in British gardens soon?"

Unlikely

> I saw a banana tree with green bananas in Switzerland

1) I believe you. This is not rare. Culturing a banana tree is super easy.

You can easily find one ornamental dwarf banana for your office in Zurich, put it outside in summer and enjoy the sight of the little green bananas. You just need lots of water, a decent sized pot (bananas can produce fruit in pots), some manure and a summer hot. All of this can be found in Switzerland.

Just don't expect fruit.

2) Growing bananas to their harvest time is the impossible part.

Flowers need to mature all winter long, and if they touch just a little chill, commercial banana fruits will stop growing, the stems will wither and collapse to the ground. They need more than 12 months of non-stop warm temperatures to became yellow and tasty. Your banana tree should be treated as an annual herb or kept indoors for many months each year.

But, there are more in this history.

There are around 50 species of banana trees. And lots of hybrids. There are yellow bananas, green bananas, lipstick pink bananas and steel blue bananas.

Some species evolved in mountains of tropical areas. Basically a temperate climate. The trunks that support the fruits can stand much more cold without dying. The problem is that they have small short bluish green bananas that don't worth the effort.

So far, those species are collector oddities or kept for hybridization purposes; but everybody in the field would salivate about breeding a seedless banana of decent quality that stands cold. This creatures are very productive.